A man can do a piece of work in 60 hours. If he takes his son with him and both of them work together, then the work is finished in 40 hours.
How long will the son take to do the same job, if he worked alone on the job?
This is the simplest of the questions you can have in work & time category. The way to solve such questions is almost similar. Try to get down to work done in 1 hr.
If the man takes 60 hours to complete the work, then, He will finish (1/60)th of the work in 1 hour.
Work done in 1 hr by Man = (1/60)th of total work
If Son takes X hours to finish the same work. Then he will finish (1/X)thof the work in 1 hour.
Work done in 1 hr by Son = (1/X)th of total work
Since they both can finish the work in 40 hrs, the work done (by both of them working together) in 1 hour is (1/40)th of the total work.
Hence, 1/40, should be same as the sum of above 2 equations..
If working alone, the son will take 120 hours to complete the work.